Not Applicable
Not Applicable
The invention relates to electronic lamp ballasts capable of driving lamps of different types, and in particular to such ballasts which automatically recognize the type of lamp by measuring electrical parameters of an installed lamp, before or after ignition.
A method which recognizes the lamp type by pre-ignition measurement involves measuring the electrode heater resistance. Applicability of this approach is limited due to the similar electrode resistance values exhibited by the majority of known lamp types.
U.S. Pat. No. 5,039,921 teaches measurement of the lamp ignition voltage. However, this method has limited applicability because several lamp types have similar ignition voltage values, and because the ignition voltage depends on lamp temperature.
Another recently proposed method involves measuring several points of the lamp I-V curve after lamp ignition. However, if a low lamp output level is required after lamp ignition, such as with lamp dimming, then initial measurement of the I-V curve at high (full) lamp output levels will result in a flash upon ignition, before the dimming occurs.
An object of the invention is to identify a gas-discharge lamp type, installed for operation with an electronic ballast, by an electrically measurable property prior to lamp ignition.
Another object of the invention is to provide a system by which lamps of different types, and in particular different power ratings, having a same lamp base arrangement and similar physical size can be used in a luminaire without user adjustment.
According to the invention, each of the lamp types usable with the electronic ballast is a type having at least one electrode heater, and the heater impedance falls within a range of impedances which is unique with respect to the others of the usable lamp types. The ballast includes a type detection circuit which measures the heater impedance while the electrodes are being heated, and a control circuit which sets the ballast operating parameters to the predetermined values for that lamp type. Where a lamp has two electrode heaters, such as a conventional fluorescent lamp, the heaters are usually connected to separate heater secondary windings on a transformer, and the impedance of the parallel combination is measured by measuring the primary winding current, or current and voltage.
In a preferred embodiment of the invention, all but one of the lamp types includes a capacitor in parallel with the electrode heater. If there are plural heaters, separate capacitors may be in parallel with each of the heaters. The capacitors have values chosen such that the absolute magnitudes of the electrode impedances fall within separate ranges for the different types.
According to another embodiment of the invention, all lamp types with which the ballast is intended to be used, in a given luminaire type, include a respective capacitor in parallel with at least one electrode heater. This embodiment has the advantage that older production lamps of the same general type, but lacking identifying impedance elements, will be identified as non-conforming so that lamp ignition can be prevented.
In yet another embodiment of the invention, a non-linear impedance element is connected to the heater, the non-linear element having the property of effecting a large change in initial heater impedance, but having lesser effect subsequently, especially during normal operation.
A first embodiment of a ballast circuit for use with such lamp types may include any well-known type of electronic arc current ballast having an arc current inverter operating at a high frequency, such as one typically between 20 kHz and 100 kHz, except that the electrode heaters are connected to separate heater windings on a high frequency heater transformer driven by a second, low power inverter. When the ballast is first energized, the low power inverter is controlled to oscillate at a predetermined frequency, and the arc current inverter is turned off. The current flowing through the heater transformer primary is then determined entirely by the electrode heating circuit load. A digital sampling circuit produces signals indicating which range the initial heating circuit impedance falls in, and the combined heater resistance. After the heaters have been adequately heated, for example when the heater resistances are four times the initially observed resistance, the arc current inverter is enabled and is controlled according to the desired parameters for the lamp type corresponding to the heating circuit impedance. For example the frequency or a combination of frequency and conduction angle of the inverter switches are controlled to provide the desired lamp power (lamp output) so that, if set by external controls for a dimming mode, the lamp does not have a bright flash before dimming to the set mode.
This embodiment has the advantage that the lamp electrode current can be controlled independent of the arc current, for example by controlling inverter conduction angle (pulse width modulation). Because the electrodes consume very little power the required inverter can be quite simple and small, and RF filtering of the electrode current will usually not be required. One model of ballast can be programmed to operate a preselected group within a wide variety of lamps. Further, the electrode current can be reduced or eliminated at high light output levels, while increasing the electrode heater current at low lamp output levels, thereby improving life time of the lamp and the efficiency of the combination.
A second embodiment of a ballast for use with such lamps requires only one inverter and transformer, but has a more complex control routine. The ballast has a resonant load circuit to which the lamp electrodes are connected, either directly or through an isolating transformer. The electrode heaters are connected either to a separate heater transformer whose primary is driven by the same inverter, or to separate heater windings on the isolating transformer. In this embodiment, when the ballast is initially energized it may be controlled to oscillate at a frequency sufficiently different from the normal operating frequency that the voltage across the arc electrodes is below that which will cause any lamp to strike. The current through the filament or isolating transformer is a measure of the heating electrode impedance. After the lamp type has been determined, the inverter frequency is set to the correct value for that lamp type so that proper ignition and desired operation can be achieved.
Where the ballast resonant load circuit has a series resonant capacitor across which the lamp is connected, the initial frequency is preferably well above the operating frequency range. This arrangement not only reduces the possibility of premature ignition before the lamp identification circuitry has completed setting the desired operating parameters, but also permits easy distinguishing between lamp types using relatively low value capacitors.
In a variation of the invention useful with multiple-lamp luminaires operated from a single ballast, separate heater transformers and identification circuits are used for each lamp. The control circuit is arranged to prevent ignition if incompatibly different lamp types, such as substantially different wattage ratings, are installed simultaneously in the same luminaire. This variation may be used with either the first or the second embodiment. However, driving a variety of lamps with the same model ballast according to the second embodiment becomes difficult, because of the fixed relationship between the electrode heater drive and the arc voltage.
The invention is useful not only with pre-heat and rapid start low pressure fluorescent lamps, but also with any other type of arc discharge lamp having at least one electrode heater and requiring a current limiting or lamp controlling ballast. The invention is also applicable whether the electrode heating is direct (filament electrode) or indirect (heater electrically insulated from the electrode).